PSI - Issue 19

Jan Presse et al. / Procedia Structural Integrity 19 (2019) 423–432 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

426

4

appeared for the hybrid joined connections with the smaller rivet (D3). The adhesive layer of the hybrid joined connections with the bigger rivet seems to be negatively affected by the rivet. Another phenomenon has been observed for adhesively bonded and hybrid joined connections. When the load amplitude undercuts a specific value, material failure in the aluminum bottom sheet appeared and led to a failure of the configuration. Therefore, the relevant range for joint failure for the LS configuration is limited to 10 4 ≤ ≤ 3 ∙ 10 5 .

10000

HYB(D5) k= 9.36 HYB(D5) HYB(D3) k= 9.66 HYB(D3) HYB Mat-Fail ADH k= 8.93 ADH ADH Mat-Fail SPR(D5) k= 6.59 SPR(D5)

R=0.1 f=10Hz

Load amplitude F a [N]

1000

1,0E+04

1,0E+05

1,0E+06

1,0E+07

Cycles to failure N [-]

Fig 2. Fatigue test results for hybrid joined LS connections.

The fatigue behavior of the investigated connections for the coach peel load case is consistent with the observed behavior from the quasi-static tests. Fig 3 shows that the fatigue strength values of each joining technique do not differ as strongly as for lap shear. The hybrid joined configurations with the larger rivet show the best fatigue performance, close to the values for the combination with the smaller rivet. Especially noticeable is that the riveted connections endure significantly more number of cycles for a higher load range than the adhesively bonded connections. This behavior seems to switch with decreasing loads so that adhesively bonded connections have superior fatigue performance under a lower load range compared with riveted connections. Additional investigations with the use of dye penetration process on hybrid joined CP specimens confirm that the adhesive layer fails in the first fifth of fatigue life up to the position of the rivet. After that process of degeneration, the rivet supports the connection by carrying a part of the load. Once a crack initiates in the top sheet steel around the rivet, a crack propagation took place for the rest of the fatigue life until separation.